Heat exchanger and vehicle air-conditioning system

ABSTRACT

The invention concerns a heat exchanger with an arrangement of parallel-guided flat tubes (1a, 1b) which are intended to conduct coolant which evaporates. Fins (2) are arranged between the tubes which form air guide slots in the direction perpendicular to the course of the tubes, and thus define the flow direction of air to be cooled. The tubes form at least two assemblies (11) which are arranged behind each other in the air flow direction (3). The assemblies (11, 12) differ in the free inner cross-section area (W1, W2) of the tubes (1a, 1b) for the coolant.

The invention concerns a heat exchanger for vehicle air-conditioning systems, with which in particular air is cooled by means of an evaporating coolant, wherein to increase the efficiency, the design of the heat exchanger takes into account the fact that the reduction in density of the coolant on evaporation leads to a reduction in the further heat absorption capacity. This is achieved in that the flat tubes form assemblies with different cross-section areas of the individual tubes for the coolant flow. In the present application, the term “heat exchanger” is used synonymously with “evaporator” or “vaporiser”.

The invention furthermore concerns a vehicle air-conditioning system.

Air-conditioning systems, in particular for vehicles, are normally equipped with heat exchangers in which a coolant, cooled and liquefied by pressure reduction, evaporates in thin pipes, whereby it extracts heat from the air passing over the pipes containing the coolant and thus lowers the temperature of the air.

For this, in the prior art a series of heat exchangers are used with different designs. In particular, round and also flat tubes are used, and serpentine tubes or tubes arranged in parallel between collectors. Typical designations in the prior art are “plate and fin”, “parallel flow”, “serpentine type” and “tube and fin”.

Heat exchangers for air-conditioning systems in vehicles must fulfil a number of requirements. In particular, they must be able to transfer up to 9 kW heating power within a short time; they must be as small as possible so that they can be arranged below the dashboard of the vehicle; the coolant flow may be up to 10 kg/min; and for the air route, the pressure fall should be as low as possible. Also, splash water protection is necessary because splash water, which occurs in particular as condensation water on the evaporator, can be transferred to the heater, where it evaporates and later condenses on the (relatively) cold windscreen, misting this up. The condensation water from the air to be cooled must therefore be securely discharged, and must not even temporarily enter the passenger compartment via the air-conditioning system.

These requirements as a whole can be considered extremely high, and in recent years and decades have led to a series of optimisations. Heat exchangers with parallel-guided flat tubes have become increasingly common because finally these are best suited to the requirements for vehicles.

US 2007/0215331 A1 discloses a heat exchanger in which the coolant is guided, falling and rising, in parallel flat tubes of the same design, wherein in the falling channels at the back, the coolant first meets the (already cooled) outlet air and only finally, in the rising channels at the front, meets the (warmer) inlet air (contra-flow process). The fact that the coolant loses heat absorption capacity on its route through the heat exchanger because of its reduction in density, is not taken into account.

The same applies to the heat exchanger in EP 0 325 844 A1. Here air flows past six assemblies of the same design, arranged behind each other. Because of the unused space between the assemblies, this arrangement is less effective and in addition also cost-intensive because of the plurality of tubes and tube connections. In order to take into account the density-dependency of the heat absorption capacity of the coolant, this arrangement could be operated such that it first passes in parallel through two assemblies with tubes, and then further on the coolant path passes in parallel through the remaining four assemblies. EP 0 325 844 A1 does not discuss such a guidance of the coolant.

DE 195 15 526 C1 discloses a heat exchanger which cools air in a total of six passes in the contra-flow process. These passes consist of a number of flat tubes of uniform design. The density-dependency of the heat absorption capacity of the coolant is taken into account here in that, in the successive passes, the number of groups of flat tubes forming the passes rises monotonously from two to five, and thus the total cross-section of the coolant flow is increased in steps, wherein the flow speed of the coolant is reduced accordingly (progressive circuiting). The disadvantage here is that, in operation with a higher pressure fall or on superheating, at the coolant outlet a smaller temperature difference from the inlet air temperature can exist in contra-flow systems than in co-flow operation.

The invention starts from this prior art and its object is to provide an additional parameter, which can be influenced for optimizing the heat exchanger and which can be used to slow down the coolant flow in the outlet region, in order thus to increase the achievable temperature difference and hence the effectiveness of the heat exchanger while retaining the same external dimensions.

This object is achieved in a first aspect of the invention by a heat exchanger with the following features.

The heat exchanger has a coolant inlet and a coolant outlet, with parallel-guided flat tubes which are intended to conduct a coolant which evaporates. Fins are arranged between the tubes which form air guide slots in the direction perpendicular to the course of the tubes and thus define the flow direction of air flowing through. The tubes form at least two assemblies through which coolant flows successively and which are arranged behind each other in the air flow direction, and these assemblies differ in the size of the inner cross-section area of the respective tubes of the assemblies. The tubes adjacent to the coolant inlet have a larger cross-section area than the tubes adjacent to the coolant outlet.

Thus, to optimise very differently designed heat exchangers, an additional parameter is provided, namely the variation in inner free cross-section area of the tubes. Also, the further knowledge is used that a division into two or a few different tube types is sufficient for practical purposes. Furthermore, due to the different tubes, the enlargement of the tube cross-section with increasing flow length can be adapted much more precisely to the lower density of the coolant than a stepped increase in the number of identical tubes as in the prior art.

Embodiments of the invention utilize this optimisation parameter to take into account, more precisely than in the prior art, the fact that during the passage of the coolant through the heat exchanger, the reducing density of the coolant on evaporation leads to a reduction in the further heat absorption capacity.

In a first embodiment of the invention, the heat exchanger has a lower and an upper collector which each consist of a base part and a cover. The coolant inlet of the heat exchanger is arranged on one of the collectors, and the coolant outlet on the same or on the other collector. The ends of the flat tubes are welded into openings in these collectors. Each of the two collectors forms either just one region or several regions separated from each other by partition walls, such that the tubes form at least two passes through which coolant flows successively, wherein at least some of the passes differ in the cross-section of the tubes and in some cases also in the number of tubes through which coolant flows. In the pass at the coolant outlet of the heat exchanger, the sum of the individual cross-sections of the tubes is greater than in the pass which starts at the coolant inlet.

By using flat tubes with a larger cross-section for the flowing coolant at least in the last pass before the outlet, it can be achieved that the pressure fall of the coolant is reduced in this region, which for a predefined outlet pressure allows a lower inlet pressure and a lower inlet temperature and hence a greater temperature difference from the incoming air. During the passage through the heat exchanger, the coolant density constantly reduces due to evaporation, and therefore a greater volume is required in order to absorb the same heat quantity (progressive circuiting). In this embodiment however, in contrast to the prior art, this greater volume is provided by tubes of greater cross-section, without—as in the prior art—more fin area being provided at the same time by increasing the number of parallel tubes, and a greater heat exchange volume being required for this.

As a whole, this optimisation not only leads to better cooling of the through-flowing air, but because of the reduced pressure drop, also to a lower dew point and hence to better drying of the air.

Optimisation calculations have shown that in this embodiment of the invention, the cross-section area of the tubes of the second assembly should be 1.1 to 2.5 times, and in particular 1.2 to 1.6 times, the cross-section area of the tubes of the first assembly.

Advantageously, additionally and supplementarily, in a further embodiment of the invention, the number of tubes of the assemblies arranged behind each other in the flow direction may differ, and hence the area of the fins assigned to the flow. In this way, the active fin area may be optimised independently of the flow cross-section of the coolant.

Preferably, all constituents of the heat exchanger are welded economically in a single work process.

In a further, particularly simple embodiment of the invention, the heat exchanger consists of precisely two passes, wherein the first pass has tubes with a smaller cross-section area and the second pass has tubes with a larger cross-section area, and wherein the tubes of each pass are formed by an assembly with tubes which have a mutually uniform cross-section. Even in this simple design, the reducing density of the coolant due to the heat supply can advantageously be taken into account in a good approximation.

In a further embodiment, the coolant flow may be optimised further if the tubes form more than two passes through which coolant flows successively. Here the flow cross-sections of the passes, calculated as the product of the number of tubes which contribute to the pass and the cross-section area of one of the identical tubes of this pass, form a monotonously rising sequence in the coolant flow direction.

Another aspect of the invention concerns the production of a plurality of heat exchangers with different nominal power. This can be optimised in that an assembly of tubes in a heat exchanger of higher performance is used as an assembly with smaller cross-section area, and the same assembly in a heat exchanger of lower performance is used as an assembly with larger cross-section area.

Other aspects concern the use of a heat exchanger according to the invention. The heat exchanger may be optimised in that the exchange of heat between the coolant and air flows takes place in co-flow, and therefore on installation the side of the heat exchanger facing the coolant inlet is facing the air inlet. If optimisation however takes place for contra-flow, on installation the side of the heat exchanger facing the coolant inlet is facing the air outlet.

The increased efficiency of the heat exchanger according to the invention in comparison with the prior art contributes in particular to fulfilling the particularly high requirements for use in air-conditioning systems in vehicles.

The drawing shows:

FIG. 1 a perspective view of an embodiment of a heat exchanger according to the invention;

FIG. 2 a cross-section through a first embodiment of the invention with two passes;

FIG. 3 an enlargement of an extract from FIG. 2 showing the different cross-sections of the tubes;

FIG. 4 a four-pass embodiment of the invention in cross-section;

FIG. 5 another four-pass embodiment of the invention in cross-section; and

FIG. 6 a cross-section through an upper collector of the heat exchanger according to FIG. 5.

FIGS. 1 to 3 show a first embodiment of a heat exchanger according to the invention for an evaporator of an air-conditioning system of a vehicle. The heat exchanger has a plurality of parallel-guided flat tubes 1—arranged vertically in FIG. 1—which are intended to conduct coolant which evaporates.

Fins 2 are welded between adjacent tubes and form air guide slots in the direction perpendicular to the course of the tubes 1, which in FIG. 1 run from the rear left to the front right. The air flows in this direction as indicated by the arrow 3, wherein however it is primarily the structure of the heat exchanger which determines whether the air flow is oriented in conformity with the layout of the heat exchanger, i.e. as shown in FIG. 1 in co-flow, or in contra-flow to the coolant flow.

Collectors 4 and 5 are arranged at the top and bottom of the heat exchanger and have openings (not shown in FIG. 1) in which the ends of the tubes 1 are welded, wherein each of the two collectors may form several regions separated from each other by partition walls (also not shown in the figure), such that the tubes form at least two passes through which coolant flows successively. The collectors 4, 5 each comprise a trough-like base part and a cover closing the respective chamber.

In the simplest case, the upper collector 4 in FIG. 1 is divided by a single partition wall into a rear region for the coolant flowing in via an inlet 8, and a front region for the coolant flowing out via an outlet 9, while the lower collector 5, as a connecting chamber of the passes, requires no partition walls. As a result, two passes are formed through which coolant flows successively, namely in FIG. 1 a rear pass in a first assembly 11 with flow direction from top to bottom, and a front pass in a second assembly 12 with flow direction from bottom to top. In general, the chambers 7 of the collectors 4 and 5 always contain firstly the openings of tubes from which coolant flows into the chambers, and secondly further inlets of tubes into which coolant flows, wherein these tubes not only include some of the flat tubes 1 but in some cases also the inlet 8 or outlet 9 of the coolant.

FIG. 2 shows a cross-section of the heat exchanger in FIG. 1. It is evident that the tubes 1, which run perpendicular to the drawing plane, form two assemblies 11 and 12 which are arranged behind each other in the air flow direction 3, namely a smaller assembly 11 at the top and a larger one at the bottom. The size of the assemblies is determined by the number of tubes 1 lying in the flow direction 3 and forming the units 15. In FIG. 2, there are eight tubes each forming a unit 15 in the first smaller assembly 11 (first in the flow direction), and eleven units 15 in the second larger assembly 12 (second in the flow direction) (see also FIG. 3).

FIG. 3 shows an extract enlargement of FIG. 2 which clarifies this further. Only here is it evident that the assemblies 11 and 12 consist of tubes 1 a, 1 b of different inner cross-section area. The assembly to which the coolant outlet 9 of the heat exchanger leads uses coolant tubes 1 a which each have a larger cross-section area W₁ than those of the assembly 12 adjacent to the coolant inlet 8 (tubes 1 b; cross-section area W₂).

The difference in cross-section is only slight. The preferred ratio of cross-section areas W₂/W₁ of the tubes of assemblies 12 and 11 is 1.1 to 2.5, and in particular 1.2 to 1.6. In the case of more than two assemblies, for each assembly a cross-section value for the tubes can be selected which lies between those of its neighbouring assemblies.

It is also evident that the width of the tubes 1 a, 1 b is the same. For structural reasons, the first and last tubes 1 a, 1 b of each unit 15 are slightly rounded on the outside. Apart from these last tubes, the other tubes 1 a, 1 b of each unit 15 have identical inner cross-section areas W₁ or W₂.

In the case of FIG. 2 and FIG. 3, the co-flow process indicated by the arrow 3 is optimised, because on average over the entire heat exchanger, greater temperature differences occur between coolant and air than with contra-flow processes. Whether contra-flow or co-flow is preferred in other configurations must be examined afresh in each case.

FIG. 4 shows an embodiment of the invention with four passes P1, P2, P3 and P4. P1 and P2 have tubes of smaller cross-section area, and P3 and P4 have tubes of larger cross-section area. This embodiment is particularly suitable for superheated operation. Here the coolant flows through four tubes, two times 1 a and two times 1 b. In this arrangement, the optimisation shows that the contra-flow process is preferred, here indicated by arrow 3 from below. Otherwise here again, the features explained in connection with FIGS. 2 and 3 apply, for example the cross-section ratios and the tube widths.

FIG. 5 shows a further embodiment of the invention with four passes P1 to P4 with successive flows, wherein P1, P2 and P3 lie next to each other transversely to the flow direction 3 and are as a whole fully covered by P4 in the flow direction. Three passes P1 to P3 coming from inlet 8 transverse the region 11 with tubes of smaller cross-section W₁, comprising twelve tubes per unit 15 in the air flow direction 3 (shown above each other in FIG. 5). The first pass P1 contains eight units 15 of tubes lying next to each other, the second pass P2 has eleven units 15 of tubes, and the third pass P3 has fifteen units 15 of tubes. The fourth pass P4 utilizes the entire region 12 with nine tubes per unit 15, and there all 34 adjacent units 15 have tubes with the larger cross-section W₂ of the tubes used. The total cross-section areas of the passes are therefore is follows:

8*12*W ₁=96*W ₁   First pass:

11*12*W ₁=132*W ₁   Second pass:

15*12*W ₁=180*W ₁,   Third pass:

and

34*9*W ₂=306*W ₂=459*W ₁   Fourth pass:

wherein for the last conversion, it is assumed that the ratio of the cross-section areas of the tubes 1 in both passes P4 and P1 to P3 is W₂/W₁=1.5. Thus the flow cross-sections of the passes in the coolant flow direction form a monotonously rising sequence in order to take into account the decreasing heat absorption capacity of the coolant: in each pass, the coolant flows more slowly than in the preceding pass, and thus compensates for the lower heat absorption capacity. Use of the entire region 12 for the fourth pass with a total of 306 tubes allows the greatest cooling to be concentrated there close to the coolant outlet, which for this embodiment leads to optimum cooling in co-flow.

The invention thus allows implementation of the knowledge that on passage through an evaporator, the gas proportion in the coolant increases and its density reduces for a constant pressure. The heat absorption capacity of the coolant is however proportional to the density, so that for a constant and effective heat transmission, an increasing volume and therefore an increasing cross-section of the coolant flow is required. In the prior art, this is achieved by means of additional channels, which however increases the dimensions of the heat exchanger by the space required for these additional channels. According to the invention, this additional space can at least partly be saved in that due to the increased tube cross-section, only additional tube volume is provided but not additional tube length and fins fitted. Heat exchangers of the same power can thus be constructed smaller, or they work more effectively for the same size. Also, the increased heat transmission at the coolant outlet, in particular with slight superheating, means that cooling may be more effective in co-flow than in contra-flow.

FIG. 6 shows an upper collector 4 of FIG. 5, the interior of which is divided by partition walls 6 into chambers 7 in order to divide the coolant flow into an increasing number in the flow direction. 

1. A heat exchanger comprising: a coolant inlet; a coolant outlet; parallel-guided flat tubes configured to conduct a coolant which evaporates; and fins arranged between the tubes which form air guide slots in the direction perpendicular to the course of the tubes, thereby defining a flow direction of air flowing through, wherein the parallel-guided flat tubes form at least two assemblies through which coolant flows successively and which are arranged behind each other in the air flow direction, and wherein the assemblies differ in a size of an inner cross-section area of the respective tubes of the assemblies, and the tubes adjacent to the coolant outlet have a larger cross-section area than the tubes adjacent to the coolant inlet.
 2. The heat exchanger according to claim 1, further comprising an upper and a lower collector between which the parallel-guided flat tubes run, wherein tubes with parallel flow form a pass and coolant flows through the passes successively, wherein the coolant inlet and the coolant outlet are arranged on one of the upper and lower collectors.
 3. The heat exchanger according to claim 2, wherein the upper and lower collectors have openings in which the ends of the tubes are tightly welded.
 4. The heat exchanger according to claim 2, wherein each of the two collectors forms a chamber or several chambers separated from each other by partition walls, such that the tubes form at least two passes through which coolant flows successively.
 5. The heat exchanger according to claim 2, wherein the passes differ in the number of tubes and/or in the crosssection of the tubes through which coolant flows, wherein, in the pass adjacent to the coolant outlet, a sum of the crosssection areas of the tubes which contribute to this pass has a greater value than in the pass adjacent to the coolant inlet by 1.2 to 1.6 times.
 6. The heat exchanger according to claim 3, wherein the number of tubes arranged behind each other in the air flow direction is different in the first and in the second assembly.
 7. The heat exchanger according to claim 1, wherein tubes form precisely two passes through which coolant flows successively, with a first pass having tubes with smaller cross-section area and a second pass having tubes with a larger cross-section area.
 8. The heat exchanger according to claim 1, wherein the tubes form more than two passes through which coolant flows successively, and in the coolant flow direction, sums of the cross-section areas of the respective passes form a monotonously rising sequence.
 9. The heat exchanger according to claim 1, wherein the tubes of one of the at least two assemblies have the same cross-section areas.
 10. The heat exchanger according to claim 1, wherein one of the at least two assemblies has a plurality of passes with successive flow, and successive passes have more tubes than preceding passes, so that a sum of the cross-section areas of the tubes increases.
 11. The heat exchanger according to claim 1, wherein several tubes are joined together into a plurality of structural units, wherein the structural units in cross-section have an oblong form and the fins are arranged between adjacent units.
 12. A vehicle air-conditioning system with a heat exchanger according to claim 1, wherein air and coolant are in co-flow and the side of the heat exchanger facing the coolant inlet is arranged facing the air inlet.
 13. A vehicle air-conditioning system with a heat exchanger according to claim 1, wherein and coolant are in contra-flow, and the side of the heat exchanger facing the coolant outlet is arranged facing the air inlet. 